fix(backfill): replay every MV feeding the target, not just the first (NUM-7480)

[alvarogar4]

Summary

Fixes NUM-7480. backfill mv_replay rebuilt a target table from only the first materialized view whose to matched it. ClickHouse allows several MVs to share one destination table, so the rest were silently dropped and the backfill came out incomplete.

• detect.ts: findMvForTarget → findMvsForTarget — returns every MV feeding the target.
• planner.ts / types.ts: the plan now carries mvReplayQueries: string[] (renamed from mvAsQuery).
• chunking/sql.ts: buildChunkExecutionSql replays all MV queries in a single INSERT INTO target SELECT … UNION ALL SELECT … per chunk, so one deterministic query_id and one insert_deduplication_token still cover the chunk (separate statements would break query-id tracking and dedup-drop every insert after the first). Single-MV plans emit byte-identical SQL.
• Threaded through both execution venues: local run (plugin.ts) and managed submit (plugin-obsessiondb).
• Docs: clarified the mv_replay strategy description for multi-MV targets.

Clean rename with no back-compat shim — @chkit/plugin-backfill is 0.1.2-beta, plans are ephemeral/regenerated and read via plain JSON.parse.

Test plan

• New unit coverage: findMvsForTarget returns all matching MVs; planner produces a 2-element mvReplayQueries + UNION ALL SQL; managed submit path does the same.
• typecheck, lint, build pass; @chkit/plugin-backfill and @chkit/plugin-obsessiondb test tasks (incl. live e2e) pass under bun verify.
• Eyeballed generated multi-MV SQL: valid ClickHouse — correct per-branch WHERE injection, preserved GROUP BYs, single trailing SETTINGS/dedup token.
• Optional: live multi-MV backfill against a real ObsessionDB service.

Note: the only bun verify failures are pre-existing live-cluster auth errors in packages/cli (untouched by this PR) — no code-path connection to this change.

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